1. Field of the Invention
The present invention belongs-to the technological field of an optical lens device having a plurality of resin lenses, and in particular it relates to an optical lens device suitable for an objective lens for an optical pickup to be used for recording or reproduction of information on or from an optical disk.
2. Description of Related Art
According to the recent trend toward a high recording density of an information recording medium such as an optical disc, an objective lens having a high numerical aperture is used for recording or reproduction of the information recording medium to meet the demand for storing a large amount of information. In the case the numerical aperture required to the objective lens is up to about 0.65, an aspheric lens produced by molding a resin can be used. However, in the case the required numerical aperture exceeds that level, the objective lens cannot be configured by a single lens, and hence it is necessary to configure the objective lens with high numerical aperture by a combination of two or more lenses. For example, the Japanese Patent Application Laid-Open under No. 2000-131603 discloses a configuration of an objective lens with a high numerical aperture by a combination of a plurality of lenses.
On the other hand, for the purpose of reducing the weight of the objective lens to enable a high speed access to recording information, a resin lens such as a plastic lens is used as the objective lens. For example, the Japanese Patent Application Laid-Open under No. 5-281496 discloses the use of a plastic lens as an objective lens for reproduction of an optical disc.
Since such a resin lens is produced by molding a resin, unevenness of the thickness and like tends to be generated in the produced lens depending on the molding method, introducing the birefringence. Therefore, in the case the resin objective lens is used as an optical pickup for a recording or reproducing device of an optical disc, due to the birefringence of the lens, the polarization state may be changed. For example, a light spot which should be circularly-polarized is elliptically-polarized. The phenomenon has not been so problematic in a conventional optical pickup using an objective lens of a relatively low numerical aperture.
However, recording or reproducing information on or from a recent optical disc of high recording density, since a small size light spot should be formed by an objective lens having a numerical aperture larger than 0.7, the birefringence problem of the resin objective lens cannot be ignored. That is, since the size and the shape of the light spot are changed due to the polarization state of the laser light irradiated on the recording surface of an optical disc through the objective, the light spot of the designed shape cannot be formed. Thereby, a problem of a high error generation rate arises in recording or reproduction of information on or from a high recording density optical disc. In particular, in the case of configuring the objective lens by a combination of two resin lenses, in order to realize a high numerical aperture and a good light collecting performance, the polarization state change derived from the birefringence becomes large so that it is difficult to realize an optical pickup with a stable performance.